Coordination chemistry of group 14 metalloles: VI. Crystal structure and dynamic stereochemistry of cis-dicarbonyl-bis(φ 4-1,1,3,4-tetramethylsilole)molybdenum and related complexes

Abstract

The crystal structure of cis-Mo(CO) 2(η 4-1,1,3,4-tetramethylsilole) 2 ( 1) shows the presence of only the uu isomer, as an enantiomeric pair (Δ, Λ). Variable temperature NMR measurements reveal the presence in solution of the Δ and Λ enantiomers (presumably uu) for the four cis-M T(CO) 2(η 4-silole) 2 complexes, M T  Mo ( 1, 6), M T  W ( 2) and cis-Mo(CO) 2(η 4-germole) 2 ( 3), but also the diastereoisomers (ΔΛ uu and trans ou) in the case of cis-Mo(CO)(PPh 3)(η 4-silole) 2 ( 5). IR spectroscopy confirms the presence of diastereoisomers for all the complexes and 31P NMR also shows this in the case of 5. The structures of the diastereoisomers are assigned on the basis of the multiplicity of the 1H NMR signals. Both rotations of the η 4 ligands and classical twists, of the type usually associated with octahedral chelate complexes, are necessary to explain this dynamic behaviour fully. The activation parameters have been calculated and the values of 30–40 kJ mol −1 for Δ G* at the coalescence temperature are comparable to those obtained for similar systems.